It is said that many bacteria live inhuman oral cavities as well as in digestive tracts in the number of species of about 300 to 400 and in the number of individuals of above 6 billions. Those bacteria live in colonies in the oral cavity to form an oral flora. Those vice bacteria in the oral cavity cause bacterial infections such as decay (caries) and periodontal diseases, which are two main diseases in the field of dentistry. Most of the bacteria are present not only in saliva but also plaques that can be said to be clumps of oral bacteria, and adhere to the surfaces of teeth and gaps between the teeth and gingivae. Among the diseases caused by the oral bacteria, attention has been focused mainly on decay (caries) and periodontal diseases. As a result, many studies have been made intensively on components of bacteriostats and bactericides for dental decay bacteria and periodontal disease bacteria. Antibacterial components for preventing decay include bactericides such as chlorhexidine, antibiotics, nonionic (non-ionic) and anionic (negative ionic) surfactants (see, for example, Patent Documents 1 to 4). However, some of the antibacterial components also have side effects such as toxicity. Some of the antibacterial components have disadvantages in that when the antibacterial components are orally taken, the antibacterial components present uncomfortable bitterness or stain the teeth or oral mucous membrane. On the other hand, recently, it has been found to utilize components of materials that are daily taken as food, such as catechin in tea and polyphenol compounds derived from various kinds of plants, as active ingredients. However, there remain problems to be solved in respect of their efficacy. For example, an unattainable amount of such the component is needed if the component is to be obtained from ordinary food. Also, there are many practical problems to be solved. For example, to increase the efficacy, it is necessary to take a specific antibody to dental decay bacteria simultaneously. Further, recently, there is a need to control not only dental decay bacteria but also oral bacteria from the viewpoints of periodontal diseases, halitosis, and oral hygiene, and there are reports on components that inhibit the growth of Porphyromonas gingivalis and Prevotella intermedia which are representative periodontal disease bacteria (see, for example, Nonpatent Document 1).
The bacteria that constitute the oral flora include both harmful pathogenic bacteria (malignant bacteria) and harmless and useful bacteria (beneficial bacteria). Known harmful bacteria in the oral cavity include Streptococcus sobrinus, Streptococcusmutans, Streptococcus anginosus, Staphylococcus aureus, and the like. Known useful bacteria include Streptococcus mitis, and the like. Streptococcus sobrinus, which is known as a dental decay bacterium, is stronger in harmness than Streptococcus mutans and is notorious as a cause of caries in the clinical field. Streptococcus anginosus is often detected from the oral cavity of patients suffered from esophageal cancer and hence the possibility is suggested that Streptococcus anginosus will be a cause of esophageal cancer. Further, Staphylococcus aureus is not only widely known as a pathogen of various abscesses but also is said to cause a complication such as pneumonia accompanying a decrease in immunity or infection with virus such as influenza, which can be a direct cause for gravity of the disease or death. On the other hand, Streptococcus mitis is a habitant that generally exists in the oral cavity and is said to occupy 80% of the oral bacteria, and is known to be an important bacterial species for forming a constant flora.
The present invention relates to an oral flora-improving agent containing a ganglioside as an active ingredient and to a drink, a food or a feed for improving the oral flora. Gangliosides are known to prevent infection by antagonistically inhibiting viruses, bacteria, bacterial toxins, and the like from attaching to trachea, or digestive tracts. However, it has not been known up to recently that gangliosides themselves have antibacterial effects. It is yet a novel finding that gangliosides have antibacterial effects against harmful oral bacteria and simultaneously growth promoting effects for useful oral bacteria. The inventors of the present invention have already filed a patent application for an invention on obtaining effects for preventing and improving periodontal diseases by using a compound having a sphingosine skeleton as an active ingredient (see Patent Document 5). The compound having a sphingosine skeleton includes a ganglioside. However, this invention is based on the finding that the compound having a sphingosine skeleton has the effect of inhibiting the bone resorption effect of osteoclasts and is effective in preventing periodontal diseases by suppressing a decrease in alveolar bone. Thus, this invention is different from the present invention that relates to the finding that gangliosides have antibacterial effect and to an oral flora-improving agent that has antibacterial effects to harmful oral bacteria and growth promoting effects for useful oral bacteria that always inhabit, simultaneously.
Recent studies revealed the problem in which the bactericides, antibiotics, and surfactants having the above-mentioned antibacterial components suppress the growth of almost all the bacteria that are present in the oral cavity therefore those bactericides, antibiotics, and surfactants are excellent in bactericidal effects, but those bactericides, antibiotics, and surfactants also break the oral flora to a greater extent. Inhibition of the growth of useful oral bacteria that are generally dominant in the oral flora makes the oral flora unstable, which will possibly lead to proliferation of harmful oral bacteria.
Therefore, instead of using a bactericide having a wide antibacterial spectrum for preventing various bacteria to kill all bacteria in the oral cavity, there is a demand for leaving useful bacteria, which provide effects desirable to living organism by allowing presence in the oral cavity, as they are to be utilized.
Further, there is a demand for an antibacterial agent that has an antibacterial effect against harmful bacteria in the oral cavity but has growth promoting effects without inhibiting the growth thereof, and is mild to useful oral bacteria.    Patent Document 1: JP-A-09-286712    Patent Document 2: JP-A-11-147814    Patent Document 3: JP-A-09-240816    Patent Document 4: JP-A-2001-064163    Patent Document 5: JP-A-2001-158735    Nonpatent Document 1: Ishikawa, Hiroki and four others, Bulletin of Japan Periodontics Society, Vol. 45, No. 1, 105-112, 2003.